The present invention relates to a method for forming a section of a sample, such as a semiconductor integrated circuit and ceramics substrate, having a plurality of conductive layers by using a focused ion beam to observe the section.
There is known, as a method to form and observe a section for observing a fine structure of a sample, a method of repeatedly irradiating a focused ion beam to a region having as a side a section observation position of a sample to form a recess (hole) in the sample as disclosed for example in JP-A-2-123749 so that a section of the sample appeared in a sidewall of the recess is observed by scanning and irradiating another charged particular beam.
FIGS. 1A and 1B show, as an example of a sample including a plurality of conductive layers, an interconnect leading to a gate of a MOSFET. Explanation will be made on a method to observe a section obtained by cutting the interconnect.
FIG. 1A is a plan view of a gate interconnect 51 portion. A source region 52 and a drain region 53 are formed on both sides of the gate interconnect 51 in a surface portion of a substrate 50. The source interconnect 55 and the drain interconnect 56 are respectively connected to the source region 52 and the drain region 53 through contacts 54. Explanation is made for a case that the gate interconnect 51 is observed, from a direction of an arrow C, in a section at a position (broken line A-B) between the source region 52 and the drain region 53. A work frame 60 (hole) is defined as in FIG. 1A, which is a region where a focused ion beam is irradiated to form a recess. Subsequently, a focused ion beam is scanned within the work frame 60 to open a hole. As a result, a recess 61 is formed as shown in FIG. 1B. The section of the gate interconnect 51 can be observed by observing in a direction of an arrow D shown in FIG. 1B.
At this time, the gate interconnect 51 (on a D side) leading to the MOSFET gate is separated from the conductive layers, such as other interconnects and semiconductor substrate, by the section formation, and is thus rendered in an electrically floating state. Because the floating-state gate interconnect 51 leading to the gate is formed on a substrate 50 surface through a gate oxide film 57, they form a capacitor. In this state, if charged particles, such as a focused ion beam or electron beam, are irradiated for observation, the charges are charged in the capacitor thus formed. A so-called charge-up phenomenon occurs wherein the potential of the floating gate interconnect 51 increases with respect to a potential of the substrate 50 when irradiating a focused ion beam, and decreases in the case of an electron beam. That is, a problem occurs because of the charge-up phenomenon when the section is observed by irradiating with a focused ion beam or electron beam in a D direction.
In particular, generally in the case of an ion beam the effected charge-up is prominent. That is, in the case of a focused ion beam, the secondary electrons are withdrawn in the section due to charge-up in the observation surface (interconnect section) to a positive potential. This makes it impossible to detect sufficient secondary electrons for obtaining an image. As a result, there is a problem in that the interconnect rendered in charge-up for the image is dark and observation is impossible.
As a countermeasure to this, there is a method disclosed for example in JP-A-7-45681 that the conductive layer in electrically floating is formed with a section separately from one for observation, an exposed new section is irradiated by a focused ion beam while blowing a metal compound gas thereby forming a metal film, and the conductive layer in floating is electrically connected to a sample substrate to avoid a charge-up. This method is effective as a means to avoid a charge-up phenomenon. However, there have been such problems that a metal compound gas blowing mechanism is required besides the focused ion beam irradiation system, and there is necessity of operations of second section formation and the succeeding metal film formation.
Meanwhile, in recent years line widths have become narrow and the gate oxide film formed between the substrate 50 and the gate interconnect 51 has become thin. In particular, the area of the floating conductive layer has been decreased by section formation due to refining in a forming region. Due to this, there has been a drastic decrease in capacitance of a capacitor formed in the process of section formation, and there has been increase in relative potential difference of the gate to the substrate due to floating phenomenon. As stated before, because the decrease in gate oxide film thickness results in a lower withstand voltage, the possibility of sample damage due to charge-up is increased. In particular, before forming a floating conductive layer in the process of section formation, the electric charges due to focused ion beam irradiating are discharged through the conductive layer. The section is observed by irradiating a scanning focused ion beam or scanning electron beam to the formed section and detecting secondary charged particles occurring from the section. However, after a recess 61 for section observation is formed and the conductive layer is rendered in a floating state, electric charges are accumulated in the floating conductive layer during formation of a portion lower than the conductive layer, causing a charge up phenomenon. There is a problem in that a high voltage is applied to the thin gate oxide film that is low in withstand voltage, resulting in damage to the sample as the case may be.